The diversification of radio communication service has increased demand for multi-band radio sets for processing signals of multiple frequency bands. In radio sets, an absolutely necessary device is a power amplifier. For efficient amplification, it is necessary to obtain impedance matching between an amplification device for amplifying a signal and a peripheral circuit of the device. Matching circuits are used for this purpose. Here, the input/output impedances of the peripheral circuit are generally set at a fixed value. Hereinafter, the input/output impedance of the peripheral circuit will be referred to as a “system impedance Z0”.
The reflection coefficient (S parameter) of the input and output of an amplification device used for an amplifier can be measured as shown in FIG. 1. Based on the reflection coefficient and the system impedance Z0, the input/output impedance of the amplification device can be measured. “S11” denotes the reflection coefficient of the input side of the amplification device and “S22” denotes the reflection coefficient of the output side of the amplification device.
The input/output impedances of the amplification device have frequency characteristics shown in FIG. 1. When an amplifier is designed using the amplification device, it is necessary to match the input/output impedances in each frequency band to the system impedance Z0.
Therefore, when designing a multi-band power amplifier, it is necessary to match the input/output impedances in each frequency band to the system impedance Z0.
Thus conventionally, when signals of multiple frequency bands are amplified, amplifiers including amplification devices and matching circuits are provided as many as frequency bands to be used and one of the amplifiers is selected according to a used frequency band as in, for example, an amplifier used in “band-sharing mobile equipment” (see, e.g., Non-patent document 1: “Mobile Equipment”, Koji Chiba et al., NTT DoCoMo technical journal, Vol. 10, No. 1). In another method, a matching circuit is designed to obtain a state close to impedance matching over used frequency bands. In still another method, some circuit constants of a matching circuit are changed.
The following will describe the method in which the circuit constants of a matching circuit are changed. The circuit constants of the matching circuit can be changed using variable devices and so on. As a matching circuit having a low loss, a matching circuit 700 of FIG. 2 is proposed which includes a main matching block 701, a delay circuit 702 having one end connected to the main matching block 701, a sub matching block 703, and a switching device 704 connected between the other end of the delay circuit 702 and one end of the sub matching block 703 (for example, see Non-patent literature 2: “Multi-band Power Amplifier using MEMS Switch”, Atsushi Fukuda et al., General conference C-2-4, the Institute of Electronics, Information and Communication Engineers, 2004).
The matching circuit 700 shown in FIG. 2 matches an impedance ZL(f) of a circuit connected to a port P2 and having frequency characteristics to an input impedance Z0 of the port P1. For example, the matching circuit acts as a matching circuit for signals of two frequency bands having frequencies F1 and F2 of FIG. 3 as center frequencies.
When the switching device 704 is turned off, the circuit of FIG. 2 acts as a matching circuit for a signal of the frequency band having the frequency F1 as the center frequency. When the switching device 704 is turned on, the circuit of FIG. 2 acts as a matching circuit for a signal of the frequency band having the frequency F2 as the center frequency. The states (ON/OFF) of the switching device 704 are switched thus, so that the matching circuit can be configured for the signals of the two frequency bands. In this case, by using, for example, MEMS technique for the switching device 704, both of a low insertion loss and a high isolation can be relatively easily obtained over a wide band, so that a multi-band power amplifier can be configured with excellent characteristics.
In the method in which amplifiers including amplification devices and matching circuits are provided as many as frequency bands to be used and the amplifiers are switched according to a used frequency, it is necessary to provide different amplifiers for the respective frequency bands to be used, resulting in a large circuit size.
On the other hand, in the method using a matching circuit designed for a wide band and the method in which the circuit constants of a matching circuit are changed, there is an advantage in circuit size reduction and so on as compared with the method of switching amplifiers.
However, when using a matching circuit designed for a wide band, it is difficult to optimally design the matching circuit for used frequency bands. Particularly, problems arise in the design of a power amplifier requiring highly efficient operations.
In the method in which the circuit constants of a matching circuit are changed with reference to FIG. 2, the matching circuit can be optimally designed for used frequencies with relative ease, achieving a high power and highly efficient operations at each operating frequency.
However, there is a problem that signals of used frequency bands cannot be efficiently amplified at the same time. To be specific, when signals of multiple frequency bands are to be efficiently amplified at the same time, the same problem as wide-band matching arises, that is, it is difficult to optimally design the matching circuit for each operating frequency. Particularly problems arise in the design of a power amplifier requiring highly efficient operations.